Using planar magnetics allows the reduction in height of magnetic components and increase in power density for state-of-the-art DC/DC converters. However, conventional structures suffer from excessive copper losses and rely on the increased spacing between windings and the core to prevent corona inception and insulation breakdown. This conventional approach has several problems.
Corona discharge and eventual insulation breakdown can be caused by voltage concentration across the air gap between the magnetic core and the printed wiring board (PWB). Insulation that supports AC voltages includes air (the gap between the core and the edge of the board) and solid material inside the PWB. When voltage is applied across two dissimilar materials such as air and a solid dielectric, material with the lower permittivity (air) will receive higher stress. The fact that voltage breakdown of air is sensitive to changes in humidity and altitude farther complicates this problem. In addition, all air gaps in the planar assembly can fluctuate due to assembly tolerances.
Interconnect vias increase component area. Individual winding turns and sections located on different layers are connected by PWB vias placed outside the immediate winding path. This arrangement requires additional area and increases winding resistance.
Added capacitance and increased winding losses can be caused by electrostatic shields. The shields reduce coupling between transformer windings thereby reducing common-mode noise currents. However, they increase transformer capacitance and eddy current losses.